1. Field of the Invention
The present invention pertains to a method of etching titanium nitride within a semiconductor structure. The method of the invention is particularly useful in etching patterned structures such as a titanium nitride gate, but may also be used for surface etchback of a titanium nitride layer.
2. Brief Description of the Background Art
The use of metal gates within semiconductor structures is a relatively new concept in the art of semiconductor manufacture. Titanium nitride is one material that is under investigation for use in gates. The use of titanium nitride as a gate material has been described, for example, by Thomas Tang et al. (International Electronic Devices Meeting Technical Digest, pp. 590-593, 1985; IEEE Transactions on Electron Devices, Vol. ED-34, No. 3, pp. 682-688, 1987); J. G. Fleming et al. (Proceedings of the 9th International Vacuum Microelectronics Conference, pp. 375-379, 1996); D. B. King et al. (Proceedings of the 9th International Vacuum Microelectronics Conference, pp. 411-414, 1996); J. T. C. Lee et al. (J. Vac. Sci. Technol. B, Vol. 14, No. 5, pp. 3283-3290, 1996); and Dong-Gu Lee et al. (J. Vac. Sci. Technol. B, Vol. 18, No. 2, pp. 1085-1088, 2000).
Titanium nitride has frequently been used in the past as a barrier layer material in aluminum metallization structures, to prevent the migration of silicon from an underlying substrate into an overlying aluminum metallization layer. Conventional chemistry for etching titanium nitride layers has been chlorine-based. Chlorine provides a very high etch rate for titanium nitride. However, a high etch rate is not always desirable when one is considering etching a very thin (i.e.,  less than 1000 xc3x85) titanium nitride layer, particularly when that thin layer is used as a gate within a semiconductor structure.
It would therefore be desirable to provide a method which provides control in the etching of thin layers of titanium nitride, and particularly control in the etching of patterned layers of titanium nitride such as in the etching of a titanium nitride metal gate within a semiconductor structure.
We have discovered a method of plasma etching titanium nitride which provides an advantageous etch rate while enabling excellent profile control during the etching of patterned structures. This method may be used alone in a single etch step, or may be used as the main etch step in a two step process, where an overetch step follows the main etch step. The two step etch process is typically used when etch selectivity of the titanium nitride to an adjacent (commonly underlying) oxide is important.
The method (or main etch step) of etching titanium nitride employs a plasma source gas comprising chlorine and a fluorocarbon gas to produce the chemical etchant species. Chlorine is the main etchant species, while the fluorocarbon serves as the secondary etchant and also provides sidewall passivation. The fluorocarbon is selected from fluorine-containing compounds having the formula CxHyFz, where x ranges from 1 to 4, y ranges from 0 to 3, and z ranges from 1 to 10. By changing the ratio of chlorine to fluorine in the plasma source gas, the etch rate can be controlled. In addition, when the etch is a patterned etch, the etch profile of the etched titanium nitride feature, such as a gate structure, can be accurately controlled. Addition of a fluorocarbon to the source gas also allows control over the amount of sidewall passivation and thus, the sidewall profile, for example, of an etched gate. The presence of the fluorocarbon also reduces the titanium nitride etch rate, permitting greater control over the etch process than when chlorine is used as the sole etchant for titanium nitride.
For example, a titanium nitride metal gate within a semiconductor structure is etched by exposing the titanium nitride to a plasma generated from a source gas comprising chlorine and a fluorocarbon. One plasma source gas which has been demonstrated to work well employs the use of Cl2 and CF4 as the chemical etchant species. Other essentially chemically inert gases may be used in combination with the chemical etchant species. In general, the method employs an atomic ratio of chlorine: fluorine ranging from about 1:10 to about 10:1; preferably, the chlorine: fluorine atomic ratio ranges from about 1:5 to about 5:1. In the instance of a titanium nitride metal gate etch, a chlorine: fluorine atomic ratio ranging from about 1:1 to about 5:1 works particularly well.
When a titanium nitride layer is etched back, without the need for submicron size patterning, it is possible to use CF4 only as the reactive etchant species. Since etched pattern profile control is not a major concern, and the main concern is control toward the end of the etching process, CF4 alone provides an overall slower etch rate, which enables better control toward the end of the etchback step. When the titanium nitride layer is thin (less than about 1,000 xc3x85), the use of CF4 alone as the reactive etchant provides a simplified process. When the titanium nitride is thicker, to an extent that etch time becomes important, it may be necessary to use a chlorine-containing main etch step and then to use solely a CF4 reactive etchant to complete the etchback. In the alternative, after the main etch step, an overetch step using the chemistry described below may be used to reach the underlying surface layer to which the titanium nitride layer is to be etched back.
In addition to the use of the etch chemistry described above, we have discovered that improved selectivity for etching titanium nitride relative to an underlying oxide layer (such as, for example and not by way of limitation, silicon oxide, silicon oxynitride, barium strontium titanate, tantalum oxide, zirconium oxide, zirconium silicate, hafnium oxide, and hafnium silicate, as well as combinations thereof), can be obtained by using a two step etch method. In the two step etch method, the main etch is carried out using a first plasma source gas comprising chlorine and a fluorocarbon, as described above (or an etch chemistry which is conventional within the art of etching titanium nitride), followed by an overetch step in which the plasma source gas comprises chlorine and bromine, which serve as the chemical etchant species. Other essentially chemically inert gases may be used in combination with the gases which provide the chemical etchant species.
In the overetch step, chlorine and bromine species are typically present at concentrations to provide a chlorine: bromine atomic ratio within the range of about 1:4 to about 4:1. This atomic ratio may be adjusted in combination with bias power to the structure which includes the titanium nitride layer to be etched. One overetch step plasma source gas which has been demonstrated to work particularly well employs Cl2 and HBr as the plasma source gases used to generate the chemical etchant species.
When the end of the main etch step is determined by endpoint, using an optical sensing technique, for example, typically, about 98% of the thickness of the titanium nitride feature (such as a gate) is etched during the main etch step, with the remainder being etched during the overetch step. When the main etch step is a timed etch, typically about 80% of the etch is carried out during the main etch step, with the remainder being etched during the overetch step.
By controlling the etch rate in the main etch step, even thin layers (i.e.,  less than 1000 xc3x85) of titanium nitride can be etched reliably. By controlling the gate etch profile in the main etch step, more devices can be placed on a given surface area of a semiconductor substrate. By controlling the selectivity of the etch of the titanium nitride gate relative to an underlying oxide layer, the gate can be etched without the danger of etch through of the underlying oxide layer.